Minimally invasive procedures using endovascular access under x-ray fluoroscopic guidance have enabled treatment of patients that previously had no options. Even in patients who may be surgical candidates, minimally invasive procedures may be preferred because of their lower risk, faster recovery, and lower cost. Among these applications are the treatment of abdominal aortic aneurysms using stents and intracranial interventions with thrombolytic and embolic agents. Guidance using x-ray fluoroscopy is well-suited for many aspects of these procedures, however it is unable to depict a number of potential complications, such as endoleaks (which occur in 20-30% of aortic stents) or intracranial bleeding, nor to fully explore hemodynamic parameters such as blood volume. Often, patients require follow-up CT scans. While tomographic imaging using C-arms is being used for imaging high-contrast objects, basic problems have prevented these systems from providing sufficient low contrast sensitivity for a number of important tasks. We propose to greatly improve the low-contrast performance of C-arm CT, and thereby provide both x-ray fluoroscopy and much higher quality C-arm-based CT images during interventional procedures. The improved C-arm CT will bring low contrast resolution approaching that of conventional CT into the interventional lab and improve image guidance for these procedures. We will address problems such as scatter, under-sampling and beam hardening, and develop hardware and software to improve low-contrast detection with the specific goal of detecting a 10 mm object of 40 HU contrast (+/-10 HU noise). We will optimize acquisition and measurement protocols for the guidance of abdominal aortic stenting and intracranial interventions, and test these in vitro and in a porcine model. We expect to demonstrate the ability to depict the relationship among a stent and the vessel wall and thrombus, to visualize endoleaks, to estimate blood volume, and to detect fresh intracranial bleeds. The significance of this project is amplified by the fact that the technology developed here could eventually impact many other minimally invasive procedures.